The present invention relates to a process for the operation of a nitrogen oxides storage catalyst with cyclical change of normalized air/fuel ratio (hereinafter called .lambda.-value) of the exhaust gas from over 1 (sorption phase) to below 1 (desorption and conversion phase) for reducing nitrogen oxides stored on the catalyst during the sorption phase and released from the storage catalyst during the desorption phase using the reductive exhaust gas constituents emitted by an internal-combustion engine during the desorption phase.
Nitrogen oxides storage catalysts were developed specially for purifying the exhaust gas from lean-burning internal-combustion engines. The class of lean-burning internal-combustion engines includes lean-burning gasoline engines, so-called lean engines, and diesel engines. Lean-burning engines, in particular with direct fuel injection, are in increasing use in motor vehicle construction since they permit a theoretical fuel saving of up to 25% compared to stoichiometrically-operated internal-combustion engines.
The principal harmful substances contained in the exhaust gas from internal-combustion engines are carbon monoxide CO, uncombusted hydrocarbons HC and nitrogen oxides NO.sub.x. In addition the waste gas also contains small amounts of hydrogen. Using modern exhaust gas purification catalysts a high percentage of the harmful substances during stoichiometric operation of an internal-combustion engine can be converted into the innocuous components water, carbon dioxide and nitrogen. Catalysts developed for exhaust gas purification of stoichiometrically operated internal-combustion engines are termed three-way catalysts.
Stoichiometric conditions exist at a .lambda.-value of 1. The .lambda.-value is the air/fuel ratio standardized to stoichiometric conditions. The air/fuel ratio indicates how many kilograms of air are needed for the complete combustion of one kilogram of fuel. In the case of conventional fuels, the stoichiometric air/fuel ratio lies at a value of 14.6.
Stoichiometric operation is maintained by regulating the air/fuel ratio offered to the internal-combustion engine. Regulation is effected by the signal from an oxygen sensor, a so-called lambda sensor, which determines the oxygen content of the exhaust gas of the internal-combustion engine. Use is principally made here of two-point sensors with jump characteristics, the sensor signal of which changes abruptly at .lambda.=1.
It is substantially harder to clean the exhaust gas of a lean-burning engine that works for the major duration of its operation with .lambda.-values greater than 1.3. Its exhaust gas contains about 3 to 15% by volume of oxygen. Heavily oxidizing conditions are consequently present in the exhaust gas. Under these conditions the nitrogen oxides in the exhaust gas can no longer be reduced in a simple manner. The cited nitrogen oxides storage catalysts have, inter alia, been developed to solve this problem. Regulation of the lean-burning operation is effected using lambda sensors with a linear dependence between the sensor signal and the oxygen content of the exhaust gas. These are termed broad band sensors. The mode of operation of two-point sensors and broad band sensors is described in the Bosch Kraftfahrttechnisches Taschenbuch, published by VDI, 20th edition dated 1995, pages 490-492.
Nitrogen oxides storage catalysts have the ability to store nitrogen oxides in a wide temperature range under oxidizing exhaust gas conditions, i.e. in lean-burning operation. This operating phase is therefore also referred to hereinafter as the sorption phase. Since the storage capacity of a storage catalyst is limited, it has to be regenerated from time to time. For this purpose the .lambda.-value of the air/fuel mixture supplied to the engine, and thus also the .lambda.-value of the exhaust gas leaving the engine, is reduced for a short time to values below 1. This is also termed enriching of the air/fuel mixture or of the exhaust gas. During this brief operating phase, reducing conditions are therefore encountered in the exhaust gas before entry into the storage catalyst.
Under the reducing conditions during the enrichment phase the stored nitrogen oxides are released and reduced to nitrogen at the storage catalyst with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen as in the case of conventional three-way catalysts. This operating phase of the storage catalyst is hereinafter also termed desorption and conversion phase. When the entire system composed of storage catalyst, oxygen sensors and engine electronics functions correctly, roughly stoichiometric conditions are present downstream from the storage catalyst during the desorption phase, i.e. the hydrocarbons and carbon monoxide present in excess upstream from the storage catalyst during the desorption phase are oxidized at the storage catalyst by the nitrogen oxides released.
A process of this kind for the operation of a storage catalyst is already known from EP 0 560 991 B1 and from EP 0 597 106 A1. According to these documents, the duration of the sorption phase is more than fifty times the duration of the desorption and conversion phase. Sorption and desorption phase are initiated by appropriate control of the air/fuel ratio. According to EP 0 560 991, the air/fuel ratio can be controlled during the sorption and desorption phase using a sensor for the air/fuel ratio. This sensor is disposed upstream from the storage catalyst in the exhaust gas stream of the internal-combustion engine. The amount of nitrogen oxide compounds adsorbed by the storage catalyst is calculated depending on the intake air and on the engine load. After a preliminarily determined set-up value for the nitrogen oxide compounds stored by the storage catalyst has been exceeded, a rich mixture is supplied to the internal-combustion engine for the desorption of the nitrogen oxide compounds. The duration of the desorption phase is set to a predetermined time between 5 and 20 seconds.
WO 97/31704 describes a process for the regeneration of a nitrogen oxides storage catalyst whereby, dependent on the operating condition of the nitrogen oxide storage catalyst, a regeneration phase is started in which a fuel mixture is delivered to the internal-combustion engine with a .lambda.-value smaller than 1. The regeneration phase is initiated when the converter emits more than a predetermined threshold quantity of NO.sub.x. The ageing of the storage catalyst is corrected by a correction in the storage efficiency factor depending on the number of already completed regeneration phases.
EP 0 690 213 A1 also describes an exhaust gas purification system that contains a nitrogen oxides storage catalyst. The exhaust gas purification system makes it possible to determine the storage capacity diminishing with increasing ageing of the storage catalyst. For this purpose the oxygen concentration of the exhaust gas is measured using an oxygen sensor downstream from the storage catalyst that delivers a linear signal depending on the oxygen concentration in the exhaust gas. For purposes of desorption of the nitrogen oxides, the air/fuel mixture delivered to the internal-combustion engine is made rich for a predetermined fixed time. During this time period, the exhaust gas downstream from the storage catalyst is initially composed stoichiometrically since the reductive components of the rich exhaust gas are compensated by the nitrogen oxides released by the storage catalyst. After all nitrogen oxides have been desorbed, the .lambda.-value of the exhaust gas downstream from the storage catalyst decreases and the linear oxygen sensor delivers an appropriate signal. The peak value of this signal during the desorption phase provides information on the storage capacity of the catalyst.
EP 0 735 250 A2 describes another exhaust gas purification system with a nitrogen oxides storage catalyst. A linear oxygen sensor is disposed downstream from the storage catalyst to determine the deterioration in the storage capacity of the catalyst. The sensor determines the amount of oxygen stored on the storage catalyst during the enrichment of the air/fuel mixture. The nitrogen oxide storage capacity of the storage catalyst can be determined using this amount of oxygen.
It is an object of the instant invention to simplify the operation of a nitrogen oxides storage catalyst as well as to distinguish permanent damage to its storage ability in order to meet the future requirements of an "on board diagnostic", abbreviated as OBD.
It is a further object of the invention to compensate for the slowly progressing damage to the storage capacity and to indicate when the storage capacity of the catalyst falls below a certain minimum value which makes exchanging of the catalyst necessary.